Design of a compact microstrip lowpass filter with sharp roll‐off using combined T‐shaped and L‐shaped resonators

Abstract

A novel compact microstrip lowpass filter with high performance is designed and analysed based on transfer function. The main resonator consists of L-shaped and T-shaped patches. The L-shaped resonator is used to increase the width of the stopband. The presented filter has −3 dB cut-off frequency at 2 GHz. It also has a wide stopband about 17.82 GHz with the suppression level of −21 dB. The transition band is ∼168 MHz from −3 to −40 dB. The locations of transmission zeros are calculated by transfer function. The presented lowpass filter has other advantages, such as high return loss more than 20 dB and low insertion loss <0.15 dB in the passband. The size of the filter is only 0.087 × 0.170 λg2, where λg is the guided wavelength at the cut-off frequency. The proposed lowpass filter has the figure of merit of 50,285, which shows its strong efficiency. The lowpass filter is simulated and fabricated and there is a good agreement between the simulation and measurement results.